Chapter 7 - The Wing

Section 4 - Wing Internals

Flap Hinge Reinforcementss

Our next task to reinforce the areas that the flap hinges are mounted.

There are three flap hinges on each side of the wing and there are hardwood reinforcements installed at each position. The flap runs the entire length of the inboard wing section, so the following discussion only involves the inboard wing.  The inboard flap hinge and outboard flap hinge attaches to both the bottom wing skin and the rear spar.  The middle flap hinge only attaches to the bottom skin. There will be three reinforcements installed into the bottom skin from the inside of the wing. The 2 reinforcements on each side of the wing should have already been installed in the rear spar during its construction.  If not we will do it here.  Cut 6 each 2.5” X 2.5” squares of ¼” aircraft plywood, and mark them as follows: left outboard bottom, left inboard bottom, left outboard rear spar, right outboard bottom, right inboard bottom, right outboard rear spar. Place an arrow on each one, this will indicate the top and point forward. 

Position reinforcement in the aft outboard corner

Position a 2.5” X 2.5” ply square in the corner of the aft bay as shown above with about 3/8” space between the rib and the aft spar.  Mark the outline with a pencil.

Aft outboard Reinforcement

Cut the inside layer of fiberglass from the bottom skin.  Be careful not to cut through the outside glass layer.  Remove the foam and test fit the reinforcement ply square.

Test Fit Reinforcement Plywood Square


Use a thick micro to install the ply square in the slot and then use 3 layers of BID over the top of the square.  Extend the BID 1.5” out on the two sides of the bottom skin and extend the glass 2 inches up the sides of the rib and the aft spar.

Completed Inboard Reinforcement

Repeat this process to install a total of four square plys into the corner where the A rib intercects with the aft spar and where the C rib intercects with the aft spar into both sides of the inboard wing section.
Cut two each 2.5” X 3” squares of ¼” plywood for the reinforcements in the bottom skin centered between the other two. These are installed in the bottom skin in a like matter against the aft spar in the aft middle bay.  Draw a lineon the inside of the aft spar as shwon below, exactly half way between the inside of the A rib and the inside of the C rib.

Position of the Center Reinforcement Ply

Center 2.5” X 3.5” ply rectangle on the line just drawn on the aft spar, space it 3/8” forward of the aft spar and draw a line around it.  The 3.5” dimension aligns with the rear spar.


Finished Middle Reinforcement Ply

Install the Ply as you did the previous ones.  Remember to extend the three layers of BID at least 2 inches up onto the aft spar.
The aft spar should have plywood reinforcement at both the inboard and outboard ends.  If you did not install the outboard reinforcement block do it now.  This is installed from the back side of the aft spar.  A 2” X 2” square of the ¼” ply is installed centered between the top and bottom of the spar and 3/8” inside of the C rib.  See the following photos.

Aft Spar


Completed Aft Spar Reinforcement

The inboard side of the aft spar was fabricated with a buried ply reinforcing.  Now we will grind the corner where the aft spar meets the A rib into a nice radius of about 3/8”.  Fill any exposed gaps and then wrap a 5 layer BID around the corner the full height of the rear spar extending 5 inches onto the A rib and 5 inches onto the aft spar.

This concludes the flap hinge reinforcements.